Previous studies have indicated that inhibitors of DNA synthesis and cell division can be isolated from various tissues. It has been shown that they have tissue specificity but not species specificity. These studies are concerned with the isolation and characterization of inhibitors of DNA synthesis and cell division from human placenta. One inhibitory substance has been highly purified and shown to be a protein. A second inhibitor has also been found that differs in action, but is, as yet, uncharacterized. Purification of the inhibitors is being done by ion exchange chromatography, gel filtration, membrane sieving, and preparative disc electrophoresis. Stabilization of the inhibitors will be effected by controlling pH, ionic strength, electrolyte content, and the presence of possible cofactors. Characterization of the inhibitor will be done by ultracentrifugation, amino acid analysis, and antibody production. A variety of cell types will be examined for responsiveness to the inhibitors. Further characterization of the inhibitors will include the determination of binding constants using an immuno-assay technique. Attention will also be directed to those substances which modify the response of susceptible cell types to the purified inhibiting substances.